Method of forming fine ceramics structure

ABSTRACT

A method of forming a fine ceramics structure having columns with a fine pattern of width and a high aspect ratio includes steps of charging a plastic mold with a ceramics slurry, solidifying the ceramics slurry, and thereafter removing the plastic mold. The plastic mold is removed by heating the plastic mold in a vacuum, employing laser ablation, employing plasma etching, or employing a solvent of low viscosity dissolving plastic mold. Especially, the plastic mold is made of acrylic, the ceramics structure is made of lead zirconate titanate, and the mold is removed by laser ablation. The resulting fine pattern ceramics structure may have columns with an aspect ratio of at least 10, and particularly a height of 100 μm and a diameter of 10 μm.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of our commonly assigned applicationU.S. Ser. No. 08/531,158, filed on Sep. 19, 1995, now U.S. Pat. No.5,676,906.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a fine ceramicsstructure, and especially fine piezoelectric ceramics columns which arenecessary for a medical ultrasonic oscillator or sonar.

2. Description of the Background Art

In general, a lost wax process is employed as one of methods for formingfine ceramics structures. This process is now described with referenceto FIG. 6.

Referring to FIG. 6, a substrate 2 coated with a resist material 1 whichis sensitive to X-rays is irradiated with synchrotron radiation (SR)through a mask 3 to be subjected to deep X-ray lithography, andthereafter developed to prepare resist structures 4 (see (1) in FIG. 6).

Then, a mold 5 is formed by electroforming the resist structures 4,which in turn are thereafter removed (see (2) in FIG. 6).

Then, the mold 5 which is formed in the aforementioned manner isemployed to carry out plastic molding, thereby preparing a plastic mold6 (see (3) in FIG. 6). This plastic mold 6 has an inverted shape of adesired fine ceramics structure.

Then, the plastic mold 6 is charged with a ceramic slurry 7, which inturn is dried and solidified (see (4) in FIG. 6).

Finally, the plastic mold 6 is baked away by heat to leave only theceramics, which in turn is fired to prepare a fine ceramics structure 8(see (5) in FIG. 6).

According to the aforementioned conventional method, it is possible toform a fine ceramic structure having depressions such as holes orgrooves of at least 100 μm in width, for example.

In the conventional method, however, it is extremely difficult to form afine ceramics structure having columns in a fine pattern of not morethan 100 μm width a high aspect ratio.

SUMMARY OF THE INVENTION

In order to solve the aforementioned problem, an object of the presentinvention is to provide a method of forming a fine ceramics structurehaving columns with a fine pattern of width and a high aspect ratio.

According to one aspect of the present invention, a method of forming afine ceramics structure is provided.

This method is adapted to form a fine ceramics structure by charging aplastic mold with a ceramics slurry, solidifying the ceramics slurry andthereafter removing the plastic mold. For removing the plastic mold, anyof a method of heating the plastic mold in vacuum, a method employinglaser ablation, a method employing plasma etching, and a method ofemploying a solvent of low viscosity for dissolving the plastic mold isemployed. Especially according to the present invention, plasma etchingis used to remove the mold.

When the method of heating the plastic mold in a vacuum is employed forremoving the plastic mold, the plastic mold is preferably made ofacrylic resin.

When laser ablation is employed for removing the plastic mold, it ispreferable that the plastic mold is made of acrylic resin, the fineceramics structure is made of lead zirconate titanate, and the laserablation is carried out by irradiating the plastic mold with a laserbeam at an energy density of not more than 350 mJ/cm².

When plasma etching is employed for removing the plastic mold, it ispreferable that the plastic mold is made of acrylic resin, and the fineceramics structure is made of lead zirconate titanate.

When a solvent of low viscosity dissolving the plastic mold is employedfor removing the plastic mold, it is preferable that the plastic mold ismade of acrylic resin and the solvent is acetone.

The inventors have carried out deep study in order to solve theaforementioned problem, to discover that the difficulty in formation ofa fine ceramics structure having fine columns of a high aspect ratio hasbeen generally caused by employment of thermal decomposition for removalof the plastic mold.

It is conceivable that the resin, which is melted in the thermaldecomposition and flows as a viscous fluid, disadvantageously pushesdown columnar projections of the fine ceramics structure. When a columnhaving a square section is placed in a fluid flowing at a certain flowvelocity, for example, its viscosity resistance is proportional toμv(h/L)/L, where v represents the flow velocity, μ represents thecoefficient of viscosity, L represents the length of each side of thesquare, and h represents the height. Namely, the viscosity is directlyproportional to the coefficient of viscosity (μ) and the aspect ratio(h/L), and inversely proportional to the length (L) of each side.Therefore, the columnar projections etc. of the fine ceramics structureeasily fall, i.e., are pushed down, as the pattern width is refined andthe aspect ratio (h/L) is increased.

According to the present invention, therefore, the plastic mold isremoved by any one of heating the plastic mold in a vacuum, laserablation, plasma etching and dissolution with a solvent.

Thus, the resin is not melted to flow for removal of the plastic mold,whereby columnar projections etc. of the ceramics structure are notbroken.

Consequently, it is possible to form a fine ceramics structure havingcolumns with a fine pattern of width and a high aspect ratio.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an exemplary apparatus for removing aplastic mold by heating the same in a vacuum in accordance with thepresent invention;

FIG. 2 illustrates the relation between one-shot laser power density andthe amount of ablation per shot;

FIG. 3 schematically illustrates an exemplary apparatus for removing aplastic mold by laser ablation in accordance with the present invention;

FIG. 4 schematically illustrates an exemplary apparatus for removing aplastic mold by plasma etching in accordance with the present invention;

FIG. 5 schematically illustrates a state of removing a plastic mold bydissolution with a solvent in accordance with the present invention; and

FIG. 6 is a sectional view showing an example of a conventional methodof forming a ceramics structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, a ceramic slurry was charged in a plastic mold and solidified asfollows, similarly to the conventional method shown at (1) to (4) inFIG. 6:

First, a conductive substrate 2 coated with a resist material 1 whichwas sensitive to X-rays was subjected to lithography with synchrotronradiation (SR) through an X-ray lithography mask 3 having a support filmof silicon nitride of 2 μm in thickness for example and a relativelythick absorbent pattern of tungsten of 5 μm in thickness for example, asshown at (1) in FIG. 6. Thereafter the substrate 2 was developed toprepare resist structures 4.

Then, the prepared resist structures 4 were nickel-plated to prepare anickel mold 5 as shown at (2) in FIG. 6, and thereafter removed.

Then, the nickel mold 5 was employed to carry out plastic molding,thereby preparing a plastic mold 6 as shown at (3) in FIG. 6. Thisplastic mold 6 was in an inverted shape of a fine ceramics structurehaving columns of 10 μmφ at a height of 100 μm. Alternatively, asubstrate may be formed by a resist plate or an acrylic resin plate tobe coated with a resist material, subjected to lithography with SR anddeveloped, for preparing a plastic mold.

Then, the plastic mold 6 was charged with a ceramic slurry 7, which inturn was dried and solidified, as shown at (4) in FIG. 6.

Finally, the plastic mold 6 was removed by any one of the following fourmethods in accordance with the present invention followed by firing,thereby forming a fine ceramics structure 8.

(1) Method by Heating in Vacuum

FIG. 1 illustrates an exemplary apparatus of removing a plastic mold byheating the same in vacuum in accordance with the present invention.

Referring to FIG. 1, this apparatus is formed by a vacuum vessel 10, apump 11 for evacuating the vacuum vessel 10, heaters 12 which arearranged in the vacuum vessel 10, and a power source 13 which isconnected with the heaters 12.

The apparatus having the aforementioned structure was employed to removethe plastic mold 6 as follows:

First, the plastic mold 6 in which the ceramic slurry 7 was charged andsolidified was arranged in the vacuum vessel 10. Then, thermaldecomposition was carried out under conditions of a temperature of notmore than 500° C. and a degree of vacuum of not more than 10⁻⁴ Torr.

At this time, the resin of the plastic mold 6 was decomposed andevaporated at extremely higher rates than those in the atmosphere whilesublimation was caused when the plastic mold 6 was made of smallmolecular weight resin, whereby it was possible to remove the plasticmold 6 without exerting force on the fine ceramics structure 8.Consequently, it was possible to form the fine ceramics structure 8having columnar projections of 10 μmφ at a height of 100 μm.

(2) Method by Laser Ablation

The plastic mold can be removed using laser ablation due to thedifference between threshold values of the fine ceramics structure andthe plastic mold in relation to ablation. In other words, it is possibleto ablate and remove only the plastic mold while leaving the fineceramics structure undamaged by using a laser power with a value betweenthreshold energy density values of the plastic mold and the ceramics. Aconcrete example is now described.

Laser ablation was carried out on a plastic mold of acrylic and a fineceramics structure of lead zirconate titanate, for example, with an ArFexcimer laser beam. FIG. 2 illustrates the relation between one-shotlaser power density (mJ/cm²) and the amount of ablation (μm) per shot.

It was possible to remove only the plastic mold through ablation withoutexerting an influence on the ceramics structure, by applying the laserbeam at an energy density of not more than 350 mJ/cm², which was thethreshold value of ceramics. Consequently, it was possible to form afine ceramics structure having columnar projections of 10 μmφ at aheight of 100 μm.

FIG. 3 schematically illustrates an exemplary apparatus for removing aplastic mold by laser ablation in accordance with the present invention.

Referring to FIG. 3, this apparatus is formed by a laser beam source 20,a mirror 21, and a scanning stage 22 for receiving and moving theplastic mold 6. When it is necessary to remove the plastic mold 6 in awide area, for example, a laser beam 23 can be applied to only anecessary area through such an apparatus.

(3) Method by Plasma Etching

When etching is carried out with a plasma of oxygen and Freon, forexample, resin is decomposed at a high rate while ceramics is etched ata low rate. It is possible to remove the plastic mold by use of adifference in dry etching resistance between the fine ceramics structureand the plastic mold. A concrete example is now described.

FIG. 4 schematically illustrates an exemplary apparatus for removing aplastic mold by plasma etching in accordance with the present invention.

Referring to FIG. 4, this apparatus is formed by a vacuum vessel 30, apump 31 for evacuating the vacuum vessel 30, an etching gas supplysource 32 for supplying etching gas into the vacuum vessel 30, and apower source 34 for generating a plasma 33.

When plasma etching was carried out on a plastic mold of acrylic and afine ceramics structure of lead zirconate titanate with plasma power of50 W and a reaction gas pressure of 0.5 Torr, for example, the plasticmold of acrylic was etched at about 3 μm/min. while the fine ceramicsstructure was not etched. Consequently, it was possible to form a fineceramics structure having columns of 10 μmφ at a height of 100 μm.

It is important to optimize plasma conditions in response to the shapeof the fine ceramics structure as formed, such as the width of and theaspect ratio the columns, for example.

The relation between RF power and the aspect ratio of the ceramicscolumns was studied as follows:

First, holes were formed in the plastic mold 6 to respectively havedifferent depths of 30, 50, 70, 90, 100, 120, 150, 170, 200, 220 and 250μm with a uniform or constant 20 μm diameter, and then the holes werecharged with a ceramics slurry, for preparing samples. These sampleswere then etched under respective conditions of RF power values of 30,50 and 100 W, to remove the plastic molds.

Consequently, it has been recognized that high ceramics columns cannotbe formed because the columns are broken similarly to the case ofthermal decomposition when the RF power is too. Table 1 shows therelation between the RF power and the heights of ceramics columns thatcan be formed, and shows an aspect ratio of at least 12.5 for a power ofat most 30 W.

                  TABLE 1                                                         ______________________________________                                        RF Power      Shape of Formable Ceramics Column                               ______________________________________                                         30 W         Diameter: 20 μm, Height: 250 μm                            50 W         Diameter: 20 μm, Height: 170 μm                           100 W         Diameter: 20 μm, Height: 90 μm                            ______________________________________                                    

The above experiment was made in relation to heights of up to 250 μm,and hence the maximum possible upper limit of the height of the ceramicscolumns achievable in the case of 30 W RF power was not determined.Formation of higher ceramics columns can be expected in the case of notmore than 30 W.

For the purpose of comparison, a plastic mold was prepared frompolyimide and subjected to an experiment. The etching rate of thissample was about 1 μm/h. under conditions of RF power of 50 W and areaction gas pressure of 0.5 Torr. Therefore, a binder contained in theceramics slurry was carbonized and hence it was impossible to obtain afine ceramics structure having columns of a high aspect ratio. Thus, itis understood that acrylic is a suitable material for the inventivemethod due to the high rate of plasma etching of the acrylic.

(4) Method by Dissolution with Solvent

FIG. 5 schematically illustrates a state of removing a plastic mold bydissolution with a solvent in accordance with the present invention.

Referring to FIG. 5, it is possible to remove only a plastic mold 6, inwhich a ceramic slurry is charged and solidified, without exerting aninfluence on a fine ceramics structure 8 by dipping the plastic mold 6in a solvent 40 of low viscosity for dissolving the resin.

When the solvent 40 was prepared from acetone in relation to the plasticmold 6 of acrylic, only the plastic mold 6 was dissolved while the fineceramics structure 8 was not dissolved. Consequently, it was possible toform a fine ceramics structure having columns of 10 μmφ at a height of100 μm.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

What is claimed is:
 1. A method of forming a fine ceramic structure,comprising charging a ceramic slurry into a plastic mold, solidifyingsaid ceramic slurry, and thereafter removing said plastic mold usinglaser ablation.
 2. The method according to claim 1, wherein said plasticmold comprises acrylic resin, said fine ceramic structure comprises leadzirconate titanate, and said laser ablation comprises irradiating saidplastic mold with a laser beam having an energy density of not more than350 mJ/cm².
 3. The method according to claim 1, wherein said plasticmold consists essentially of acrylic resin.
 4. The method according toclaim 3, wherein said fine ceramic structure consists essentially oflead zirconate titanate.
 5. The method according to claim 1, whereinsaid fine ceramic structure consists essentially of lead zirconatetitanate.
 6. The method according to claim 1, wherein said ceramicslurry is a lead zirconate titanate slurry.
 7. The method according toclaim 1, further comprising a preliminary step of preparing said plasticmold of acrylic resin, wherein said preliminary step comprisespatterning a resist structure with a negative pattern using synchrotronradiation lithography, forming a nickel mold with a positive patterninversely corresponding to said negative pattern by plating nickel ontosaid resist structure, and then charging said acrylic resin into saidnickel mold to form said plastic mold to have a final mold patterncorresponding to said negative pattern.
 8. The method according to claim1, wherein said fine ceramic structure consists essentially of saidsolidified ceramic slurry remaining after said plastic mold is removed.9. The method according to claim 1, further comprising preparing saidplastic mold and carrying out said laser ablation so that said fineceramic structure is formed by said solidified ceramic slurry remainingafter said plastic mold is removed and so that said fine ceramicstructure includes at least one ceramic column having a height and awidth with an aspect ratio of at least
 10. 10. The method according toclaim 9, wherein said height is at least 100 μm.
 11. The methodaccording to claim 1, wherein said plastic mold is prepared and saidlaser ablation is carried out such that said fine ceramic structure isformed by said solidified ceramic slurry remaining after said mold isremoved so that said fine ceramic structure includes at least oneceramic column having a height of at least 100 μm.
 12. The methodaccording to claim 1, wherein said laser ablation is carried out so asto ablate and remove only said plastic mold without ablating saidsolidified ceramic slurry.
 13. The method according to claim 1, whereinsaid laser ablation is carried out using a laser beam having an energydensity that is between a first ablation energy density threshold of aplastic material of said plastic mold and a second ablation energydensity threshold of said solidified ceramic slurry.
 14. The methodaccording to claim 1, wherein said laser ablation is carried out using alaser beam having an energy density greater than a threshold energydensity needed to ablate said plastic mold and not more than 350 mJ/cm².15. The method according to claim 14, wherein said laser beam is apulsed laser beam, and said energy density of said laser beam is a oneshot power density of one pulse of said pulsed laser beam.
 16. Themethod according to claim 15, wherein said laser beam is an ArF excimerlaser beam.
 17. The method according to claim 1, wherein said laserablation comprises applying a laser beam to only said plastic mold andnot to said solidified ceramic slurry by directing and scanning a laserbeam at and along said plastic mold.